1. Field of the Invention
The present invention relates to a measuring method of a refractive index and a measuring apparatus of a refractive index.
2. Description of the Related Art
Japanese Patent Laid-Open No. (“JP”) 11-344313 proposes a method for fixing a test object between two transparent plates via intervals among them, for measuring an optical path length between the transparent plates, an optical path length between the transparent plate and the test object, and an optical path length in the test object, and for calculating a refractive index of the test object based on the measurement results. JP 02-008726 proposes a method for measuring a refractive index of a test object by immersing a glass sample having a known refractive index and a known shape and the test object in each of two types of matching oils having different refractive indices.
The method disclosed in JP 11-344313 has difficulties in measurements when the test object, such as a lens having a curved surface, decenters or tilts relative to the optical axis, and needs time-consuming strict centering or slope adjustments. Otherwise, measured value would contain errors due to the influence of the curvature of the reflected surface.
The method disclosed in JP 02-008726 needs time-consuming adjustments of the refractive index of the matching oil due to blending of different types of oils. In addition, in measuring a transmitted wavefront of the test object having a high refractive index in accordance with the method of JP 02-008726, a detector can output only a weak signal and the measuring precision becomes low, because the matching oil having a high refractive index has a low transmittance.